In co-pending U.S. patent application Ser. No. 419,705 filed on Sept. 20, 1982, a technique was disclosed that enables an optical designer to select compatible optical materials for the refractive elements of an optical system that is to be color-corrected at a specified number of wavelengths. A technique disclosed in co-pending U.S. patent application Ser. No. 260,106, filed on Oct. 20, 1988, enables the designer of a lens system that is to be color-corrected at a specified number of wavelengths to include one or more liquid lens elements among the lens elements of the system.
In co-pending U.S. patent application Ser. No. 282,665 filed on Dec. 12, 1988, particular examples were disclosed of apochromatic lens triplets of a type in which a liquid lens element is contained between two coaxially positioned and geometrically identical glass lens elements facing in opposite directions. The advantage of a lens triplet design in which a liquid lens element is contained between two identical glass lens elements becomes especially significant for mass-production manufacturing.
For the apochromatic lens triplets disclosed in the aforementioned U.S. patent application Ser. No. 282,665, both surfaces on each of the two glass lens elements of each lens triplet are spherical. Surface sphericity of lens surfaces is highly desirable in an optical prescription for glass lens elements, because the process of grinding (or chemically etching) and polishing a glass lens surface to a specified aspherical configuration is more complex (and hence more costly) than the process of producing a spherical lens surface. In general, there is no incentive in the optical design art to use an aspherical lens surface instead of a spherical lens surface, unless a significant performance advantage inherent in a particular aspherical surface is known beforehand that would compensate for the complexity and cost of producing the aspherical surface.